1. Field of the Invention
The present invention relates to a thin film transistor (TFT) substrate for use in a liquid crystal display (LCD) device and a method of manufacturing the TFT substrate. More particularly, the present invention relates to a TFT substrate for an LCD device having improved corrosion resistance, and a method of manufacturing the TFT substrate.
2. Description of the Related Art
A display device, in general, converts electrical signals into images perceivable by a viewer. Types of display devices include: a liquid crystal display (LCD) device; an organic light emitting display (OLED) device; and a plasma display panel (PDP). Arrangement of liquid crystals of an LCD device is varied in response to application of an electric field, and light transmittance of the liquid crystals is changed to display an image.
Gate resistance of a thin film transistor (TFT) used in a large screen LCD device is kept small to prevent a delay of the electric signal which would be seen as a flicker of the image. Examples of metals used for gate lines in an LCD device include copper and aluminum. However, use of copper and aluminum presents problems in the manufacture of the LCD devices. The copper has a lower electrical resistance than the aluminum. The adhesive strength between the copper and a substrate is small, and additionally copper is corrosive. Also copper easily diffuses into a silicon substrate.
A material for the line having improved ohmic contact characteristics has been developed to improve characteristics of the TFT. A conventional gate line has a double layer structure of AlNd/Cr. The aluminum has various characteristics such as low resistance and high electrical conductivity. However, the adhesive strength of aluminum is small, and aluminum is corrosive.
Corrosion of the aluminum line is divided into two types which include galvanic corrosion and an electrolytic dissolution. Galvanic corrosion is formed between two different metals. The electrolytic dissolution is formed in an electrolyte. Corrosion deteriorates the lines of the LCD device.
In order to reduce the corrosion of the aluminum line, a passivation layer such as silicon nitride film is deposited on the line. However, if a pin hole forms in the passivation layer, electrolytic dissolution is formed through the pin hole. Pin holes are formed as the result of evaporation of impurities during the deposition of the silicon nitride film, or cracks of the passivation layer due to the step difference of the underlying layer. Electrolytic dissolution can quickly result in a short of the line.
FIG. 1 is a cross-sectional view showing a defect of a silicon nitride passivation layer on an aluminum layer.
Referring to FIG. 1, a chromium layer pattern 111 and an aluminum/neodymium alloy layer pattern 112 are on a substrate 100. A silicon nitride layer 120 is stacked on the aluminum/neodymium alloy layer pattern 112. A pin hole 121 has formed in the silicon nitride layer 120 so that the aluminum/neodymium alloy layer pattern 112 is partially exposed through the pin hole 121. The exposed aluminum/neodymium alloy layer pattern 112 is corroded.
Conventionally, in order to prevent the corrosion of the line, an aluminum oxide layer is formed on the line using aluminum anodizing process through an electrochemical treatment. The aluminum oxide layer resists a voltage applied to a data line. An aluminum oxide layer having a compact structure has a growth speed of about 14 Å/V. A thickness of an aluminum oxide layer that resists a voltage of more than about 5V is about 80 Å to about 100 Å. The aluminum oxide layer is formed using a non-aqueous solution. A conventional non-aqueous solution for the aluminum oxide layer includes ethylene glycol of about 89 wt %, water of about 10 wt % and ammonium tartarlate or ammonium salicylate of about lwt %.
In the electrochemical method, the oxide layer is formed before patterning of the line or after patterning of the line. When the oxide layer is formed before the patterning of the line, the oxide layer is not easily patterned. In addition, when the oxide layer is formed after the patterning of the line, the aluminum neodymium alloy layer is easily detached from the chromium layer, and an additional process of partially etching the oxide layer for electrically connecting an indium tin oxide layer is required. Furthermore, an additional power supply is required to apply an electric potential to the substrate in a solution, thereby increasing the manufacturing processes required to produce the LCD device.
Suitable techniques for forming the passivation layer are described in references such as: (1) Y. T. Tao, G. D. Hietpas and D. L. Allara, Self-assembled Monolayers of n-alkanate, Journal of American Chemical Society, 118, 6724 (1996); (2) M. A. Purinin, A. P. Nazarov and Y. N. Mikhailovski, Oxyalkylsiloxane Thin Film on the Aluminum Oxide Layer, 143, 251 (1996); (3) J. S. Osenbach and J. L. Zell, Silicon-Based Polymers, IEEE Transactions Components Hybrids Manufacturing Technology, 1.6, 350 (1993); (4) H. W. White, C. D. Crowder and G. P. Alldredge, Production of a Protective Thin Film on Aluminum Using Aqueous Solution of Methyl Phosphonic Acid, Journal of Electrochemical Society, 132, 773 (1985).
In the above-mentioned methods, a non-metal passivation layer is formed on the aluminum layer. However, when a high voltage of about 3V to about 5V is applied to the line, the non-metal passivation layer does not resist the high voltage. In addition, the additional patterning process is required to partially remove the non-metal passivation layer.